Electronic or static electricity meters are known, for example from Bull, SEV 62 (1971) 7, pp. 371-377, translated into English in Landis & Gyr review 19 (1972) pp. 9-16. These type of meters have a high precision over a wide range of a load. The use of meters of this type entails a risk however, that a malfunction of one of the electronic elements used in the meter may result in a large measurement error, or could even result in the meter no longer functioning.
From German laid-open patent specification DE-OS 27 38 117 it is known to recognize such errors by feeding reference signals of predetermined amplitude and frequency into the electricity meter, either simultaneously or alternately with the measurement signals, and wherein the product of the reference signals is compared with the actual product obtained from the measurement signals. If the reference signals are processed simultaneously with the measurement signals, then it is expensive to separate the product of the measurement signals from the product of the reference signals. If, however, the reference signals are fed to the meter alternately with the measurement signals, then undesired interruptions of the measurement signals occur. In both of the afore cited cases, as a result of feeding the reference signals directly into the circuit of the electricity meter, the accuracy of measurement may be impaired.